1. Field of the Invention
The present invention relates to novel inorganic pigment compositions based on the rare earth metal sulfides, and, more especially, to novel pigment compositions comprising specific rare earth metal sesquisulfides and transparent oxides in the form of particles of core/shell type, the sulfides constituting the core and the transparent oxides the shell or sheath coating therefor.
The present invention also relates to a process for the preparation of such novel compositions and to the use thereof for the coloration of various materials and substrates.
2. Description of the Prior Art
Inorganic colorants/pigments are currently widely used in numerous industries, especially in the paints, plastics and ceramics industries. For such applications, their various properties, notably intrinsic color, coloring power and opacifying power, thermal stability, chemical stability, dispersibility (ability of a product to disperse properly in a given medium), and the absence of toxicity, constitute particularly important criteria to be taken account of in the selection of a suitable pigment.
The rare earth metal sesquisulfides, of general formula M.sub.2 S.sub.3 in which M represents at least one rare earth metal, are known compounds and have been described in numerous publications.
Moreover, their use as colorants/pigments for the coloration of various materials, such as, for example, plastics, paints and others, is described, in particular, in EP-A-0,203,838, assigned to the assignee hereof and hereby expressly incorporated by reference.
However, for this application, it transpires that the rare earth metal sesquisulfides hitherto known to this art display characteristics, especially chromatic characteristics, as well as a stability, both thermal and chemical, which remain insufficient and militate against any meaningful development thereof.
Another difficulty is presented by the fact that most of the synthesis processes for preparing rare earth metal sesquisulfides of high phase purity entail reactions of the solid (rare earth metal compounds such as oxides)/gas (sulfurizing agents such as H.sub.2 S) type under pressure and/or temperature conditions which are incompatible with operation on an industrial scale. In addition, certain parasitic phases are typically formed consisting especially of rare earth metal oxysulfides which may mask or degrade the intrinsic color of the desired pure sesquisulfides.